This invention relates to an ink composition for ink jet printing, and more particularly to an aqueous ink composition for use in an ink jet printing system of a type which involves projecting an ink from a small nozzle by rapidly or instantaneously reducing an inner volume of an ink-holding container.
An ink jet printing system of the type mentioned above is known, for example, in British Patent Specification No. 1,350,836. Broadly speaking, the jet system of the British patent specification includes the following two steps: first, an inner volume of a small container filled with an ink is rapidly or instantaneously reduced by means of an electric driving pulse to force the ink droplet to be projected from a nozzle tip so that one ink droplet is deposited on recording paper every driving pulse (first step); and after completion of the jetting, the system is entirely then returned to an initial state and stands ready for a subsequent jet operation (second step).
Ink compositions to be employed in the ink jet printing system of the class just mentioned should meet at least the following characteristic requirements similarly to inks for use in other types of ink jet recording systems:
1. Being free from clogging in the nozzle. PA1 2. Ensuring a sharp contrast upon recording. PA1 3. Neither changing in physical property nor producing any precipitates during storage.
In addition, it is found that the ink for use in the ink jet system of the above-mentioned type is essentially required to have suitable ranges of a viscosity and a surface tension so as to attain a satisfactory ink jet recording. Particularly, the viscosity of an ink gives a great influence on the recording. For example, when an ink is used having a viscosity outside a suitable range which is usually determined depending on the type of an employed apparatus and the kind of electric driving pulses to be input, the above-mentioned first and second steps do not proceed smoothly by the reasons described hereinbelow, it being thus difficult to effect the ink jet recording satisfactorily. That is, with an ink having a viscosity below a suitable range, it is almost impossible in the first step to correct a speed variation or deviation in the respective portions of an elongated ink droplet being ejected from a nozzle tip. As a consequence, two or more ink droplets with different velocities are undesirable projected in correspondence with one electric driving pulse. If the nozzle tip is positioned at a relatively great distance from a recording medium, the two or more droplets are deposited on the recording medium in different points to form corresponding different dots thereon, lowering the quality of the resulting print or image considerably. Further, an ink having a viscosity below a suitable range also impedes a smooth operation in the second step. That is, after completion of the jetting of the ink droplet, an ink meniscus is temporarily lowered by the reaction of the ink jetting. The meniscus then rises or is returned to an original state by the action of capillary force. In this connection, however, if the ink viscosity is too low, the ink does not satisfactorily serve as a damper. Thus, it undesirable takes a relatively long period of time until the ink meniscus settles down in an equilibrium position. This is because the ink meniscus will pass over a level of the nozzle tip and be then pulled down by the action of surface tension force, repeating the vibration damping type behavior so as to restore a standstill position thereof. On the contrary, with an ink having too high viscosity, it does not show any vibration damping type behavior, but requires a long period of time before being pulled back by the action of capillary force. In either case, the returning to an initial or ready-to-jet state of the ink takes an unfavorably long period of time, lowering a printing speed.
A number of ink composition for ink jet printing have been heretofore proposed but most of them fall short of one or more of the above-mentioned requirements and have thus inherent disadvantages which must be overcome. For example, there is known an ink composition to which is added a water-soluble thickener so as to maintain a viscosity of the composition at a desired viscosity. However, the water-soluble thickener which includes, for example, a cellulose derivative such as hydroxylpropyl cellulose, carboxylmethyl cellulose, hydroxylethyl cellulose, methyl cellulose or the like, or polyvinyl alcohol tends to change a viscosity of the ink composition or, in some cases, to produce precipitates during storage in combination with a water-soluble dye or coloring agent. This is practically disadvantages in the application to the ink jet system in which the change in the viscosity exerts a detrimental influence in carrying out the process of the system. In the ink jet system, it is general to use a nozzle with a diameter as small as 30 to 200.mu. . With such fine nozzle construction, the nozzle is readily clogged due to precipitation of solid matter resulting from evaporation of solvent when the composition is present or stayed in the nozzle. Further, the air corresponding to the amount of an evaporated solvent may enter a nozzle tip and remain in a head as bubbles. This will impede a sharp ink jet response to an inner volume change caused by an electric driving pulse. As a result, no injection of an ink takes place, or the injection conditions or state varies with a lapse of time, so that it becomes difficult to effect the recording in a stable state.
Moreover, the U.S. Pat. No. 3,705,043 discloses an aqueous ink composition which includes 1 to 8% by weight of a water-soluble dye and 5 to 30% by weight of a wetting agent such as polyethylene glycol, polypropylene glycol, ethylene glycol, propylene glycol, diethylene glycol, glycerol or the like. However, such small amount of the wetting agent is not sufficient to maintain the composition in a viscosity range required for the intended ink jet system. In addition, even with the aqueous ink composition, it is hard to prevent both the nozzle from being clogged and the air from entering the nozzle tip.